1. Field of the Invention
The present invention relates to a battery pack having a communication function, and an electric device using the battery pack.
2. Description of the Background Art
It is known that a secondary battery, particularly, a sealed secondary battery may cause a hazardous condition such as leakage of an electrolyte or emission of an inflammable gas, if the secondary battery is overcharged, overdischarged, or reverse charged in excess of an adequate amount. In view of this, generally, a battery pack with a built-in secondary battery has a battery controller for controlling a current circuit switching device such as an electromagnetic contactor or a semiconductor switching device to monitor a condition of the battery, and to suspend charge/discharge of the battery by opening a charge/discharge circuit; or for requesting an electric device of an appropriate charge/discharge by communicating a battery condition to the electric device. In particular, most of the battery packs with built-in lithium secondary batteries are provided with a battery controller having the above arrangement in view of a likelihood that a hazardous condition may occur in overcharge or overdischarge. Generally, in the case where a battery controller is provided with a communication function to communicate with an electric device, a microcomputer is loaded in the battery controller. In many of the cases, an electric power for driving the microcomputer or a like device is supplied from the secondary battery provided in the battery pack (see e.g. Japanese Unexamined Patent Publication No. Hei 8-083627).
Also, it is a general practice that the battery controller is operative to send, to the electric device, information indicating the condition of the secondary battery provided in the battery pack by communicating with the electric device using the battery pack so that the electric device can properly use the battery pack based on the battery condition information (see e.g. Japanese Unexamined Patent Publication No. Hei 11-341689).
The battery packs are used with a variety of kinds of electric devices, and different output voltages are required for the battery packs. In view of this, the battery packs are individually manufactured, in which the number of series-connected secondary batteries to be loaded in the battery packs is changed in accordance with a voltage required for the individual electric devices.
As mentioned above, in the case where battery packs with different output voltages are manufactured for the electric devices, in which different power source voltages are required, the number of the kinds of battery packs is increased, and it is necessary to design and develop battery cases, battery controllers, or like devices for the different kinds of battery packs. This may increase the cost relating to designing/development, and extend the time required for designing/development. In view of this, there is proposed an idea of setting the output voltages of the respective battery packs to a fixed value, and connecting the battery packs to an electric device by series-connecting the battery packs of the number corresponding to a voltage required for the electric device. This may be advantageous in reducing the cost relating to designing/development, and the time required for designing/development, considering a merit that various electric devices are operable by use of the battery packs of the same kind.
In the conventional arrangement, however, use of the electric device, with the battery packs being connected in series to the electric device, may cause a short-circuit between the secondary batteries through a communication circuit in communicating with the electric device, and render the electric device inoperable, because ground potentials of power suppliers of the battery controllers are different among the battery packs. FIG. 6 is a diagram for describing how a short-circuit occurs in the case where the battery packs according to the background art are connected in series to the electric device.
The battery pack 101 shown in FIG. 6 has a secondary battery 110 and a battery controller 111. The battery controller 111 is constituted of e.g. a microcomputer with a built-in communication circuit 112. A wiring 113 connected to the positive electrode of the secondary battery 110, and a wiring 114 connected to the negative electrode of the secondary battery 110 are adapted to supply a power source voltage for operating the battery controller 111 and the communication circuit 112. A battery pack 102 has an arrangement identical to the arrangement of the battery pack 101.
An electric device 103 operable by connecting the battery packs 101 and 102 in series is provided with a communication circuit 131 for communicating with e.g. the battery packs 101 and 102, and a control circuit 132 for performing an adequate control based on battery information received by the communication circuit 131. A communication wiring 133 for transmitting a communication signal, and a communication wiring 134 for transmitting a reference potential of the communication signal are connected between the communication circuit 112 of the battery pack 101, and the communication circuit 131. A communication wiring 135 for transmitting a communication signal, and a communication wiring 136 for transmitting a reference potential of the communication signal are connected between the communication circuit 112 of the battery pack 102, and the communication circuit 131. Further, a wiring 137 for connecting the battery pack 101 and the battery pack 102 in series is provided.
In the above arrangement, as shown in FIG. 6, when the battery packs 101 and 102 are connected to the electric deice 103, the reference potential of the communication signal is set to the reference potential of the power source voltage for operating the communication circuit 112 i.e. the negative electrode potential of the secondary battery 110. Accordingly, as shown by the short-circuit path X, the secondary battery 110 of the battery pack 102 is short-circuited from the positive electrode of the secondary battery 110 of the battery pack 102 via the wiring 137, the wiring 114 of the battery pack 101, the communication circuit 112 of the battery pack 101, the communication wiring 134, the communication circuit 131, the communication wiring 136, the communication circuit 112 of the battery pack 102, and the wiring 114 of the battery pack 102 to the negative electrode of the secondary battery 110 of the battery pack 102. Thereby, the series-connected battery packs 101 and 102 are rendered unusable.